The invention relates to a device for the indirect contactless electrical measuring of short distances, and particularly to a device for measuring the stroke and/or the position of nozzle needles for fuel injection nozzles of internal combustion engines.
A real-time measurement of the stroke and/or of the position of the nozzle needle of the injection nozzle is required in order to ensure optimum electronic control of the injection of fuel in a diesel motor. Thus, a generic device is known (German Unexamined Published Patent Application No. 2,949,705) in which a control element, which consists of soft-magnetic material and is moved by the nozzle needle and serves as coil core, is displaced in an excited differential coil that is concentrically fixed with respect to the latter. The differential coil, which consists of two coil portions, is here connected as a halfwave bridge. The inductive voltage in one coil portion changes with a movement of the nozzle needle, is picked up as bridge voltage between the two coil portions, and is fed to a second halfwave bridge located in an electronic control unit where it is correspondingly evaluated as a measure of the path travelled by the nozzle needle. Apart from the low sensitivity, a disadvantage of this device is the risk that the control element, while serving as coil core, will break off as a result of transverse vibrations or will be destroyed by contact with stationary parts. An advantageous feature, however, is the low temperature-dependence of the measurement.
The same applies to another known device (German Patent No. 1,049,635) in which a coil core pin of soft iron, which is moved by the nozzle needle and is passed through the differential coil consisting of two coil parts, is displaced in the coil and thereby influences the balance of inductance in the coil which is correspondingly evaluated via a measuring bridge.
A device is also known (German Unexamined Published Patent Application No. 2,920,669) which operates with a "Hall" generator. In this arrangement, a small permanent magnet is mounted in the plunger loaded by the nozzle needle, the field of the magnet is detected by a stationary hall generator arranged in the nozzle holder. When the nozzle needle moves, the permanent magnet is moved with respect to the Hall plate which changes the magnetic field passing transversely through the Hall plate, which leads to a change in the Hall voltage. A disadvantageous feature in this device is the low sensitivity of the Hall generator and its temperature dependence. An advantageous feature is the low mass of the permanent magnet to be moved and the absolute contactlessness of the measurement.
One object of the present invention is to provide a device which has both a large operating temperature range and high sensitivity but simultaneously is reliably constructed so that it can meet the requirements particularly of applications in a rough environment (oil, high temperatures, rapidly occurring movements, strong vibrations).
The core of the coil consists of an amorphous alloy which is characterised by low hysteresis and eddy-current losses, high permeability, and low coercive field strength in order to be able to keep the dimensions of the device and the number of coil turns small.
In contrast, the core of the control element consists of either a cobalt-samarium alloy in order to achieve small dimensions and low mass or, of alnico in cases of particularly high demands on the temperature stability of the magnetization. The core of the control element may also be a ferrite material.
The device of the present invention operates to counter temperature effects on the permanent magnetic core parts, the soft magnetic coil core, and the coil itself in optimum manner, and thereby possess a low temperature dependence.
According to the present invention, a device for indirectly measuring a short distance includes a housing having a longitudinal axis, a measurement coil assembly, a control element, and means for detecting a change in the inductance of the measurement coil assembly. The measurement coil assembly includes a first coil having opposite ends, a companion first core of a soft-magnetic alloy, and a measurement face defined by one end of the first coil. The control element includes a permanent magnet and a measurement face defined by the permanent magnet. The control element is movable along the longitudinal axis of the housing to present the measurement face of the control element in axially spaced confronting relation to the measurement face of the first coil such that movement of the permanent magnet in relation to the first coil causes a change in the magnetic flux density in the first core to effect a change in the dynamic permeability of the first core and thereby change the inductance of the coil. The detecting means is operable to recognize a change in the inductance of the first coil to indicate the distance between the measurement face of the control element and the axially spaced apart measurement face of the first coil.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.